The introduction of the zebra mussel (Dreissena polymorpha) to North America in 1986 has had a dramatic impact on water users throughout the continent. This organism has colonized the entire Great Lakes system, the Mississippi River and associated tributaries as well as inland waterways throughout North America. Another species of mussel, the quagga mussel (Dreissena bugensis) was introduced in the 1990's and has also spread throughout North America.
Although they are separate species they share a trait which has been devastating to raw water users throughout the continent. Mussel veligers (initial offspring) reach a stage in their maturation wherein they settle and attach themselves to hard surfaces. This is particularly troublesome for industrial water users as service water piping provides an ideal surface for attachment. Resulting problems include reduction of pipe bores and associated hydraulic carrying capacity, enhanced electro-corrosion of pipe materials and increased maintenance costs associated with the accumulation of shell debris. Since the early 1990's, industry has spent millions of dollars searching for mitigation methods which are reliable, cost effective and safe.
Mussel infestation is a particularly sensitive matter when considering water systems that are static or semi-static in nature, such as closed-loop cooling or fire protection systems. Fire protection piping at industrial facilities consists primarily of buried piping networks; although some contain sections of exposed piping. Distribution networks connect fire water supply pumps to fire protection outlets such as hydrants, automated sprinkler systems, and water spray/mist systems specifically designed for fire suppression during an emergency.
These systems are often characterized by low volumes of make-up water added continuously to maintain system pressure due to loss through leakage, or use through other purposes such as maintenance, washing, cooling, flushing or training. As a result, fresh, oxygenated water is sporadically introduced into the system replacing stagnant water. This exposes fire protection and other static and semi-static water systems to an increased risk of infestation by bivalves such as zebra mussels. In this way, mussels are able to colonize intermittent flow systems, become established, and proliferate as a result of sporadic infusions of fresh water. This poses a serious safety issue—even small numbers of mussels can cause problems in fire protection systems if they are flushed downstream during emergency use since, they can block or significantly reduce flow in the extremities of the distribution network (e.g., the narrower diameter piping and sprinkler nozzles).
Semi-static water systems have been difficult to effectively treat for mussel colonization using traditional methods, such as oxidants and specifically chlorination. Chlorination is non-selective and presents significant operational challenges in semi-static and static water systems mainly due to a characteristic of chlorine chemistry commonly known as “demand”. Demand is the potential of a given water system to consume applied chlorine (via slime, algae, chemical reactions etc.) leaving little or no residual chlorine remaining to control the target species. In systems where the flow is intermittent, demand is a major factor which will render chlorination ineffective.
The only available option to overcome this problem is to constantly flow treated water through these systems so that the target chlorine residual can be replenished and maintained. When using chlorination for control, mussels generally need to be exposed to the chemical on a continuous basis for several weeks to achieve complete mortality and only under warm water conditions, e.g., >15° Celsius. Therefore, the volume of water required to be flushed through the system would be substantial and, in many cases, it is not practical for the industrial water user to use chlorination due to the large volumes of water involved. In addition, chlorination is even less attractive due to its high oxidation potential and associated risks including the production of undesired by-products, and the necessity for end-of-pipe treatment to mitigate environmental impacts and to comply with environmental regulations.